Photonic apparatus for controlling polarization

ABSTRACT

A photonic device has a polarization-dependent region and a device layer including a first cladding film, a second cladding film, and a core film. The core film includes one of (1) a material having an index nM and (2) alternating layers of a first material having a first index and second material having a second index. The alternating layers have an effective index for TE polarized light nTE and an effective index for TM polarized light nTM. Each of the first cladding film and the second cladding film include the other of (1) the material having the index of refraction nM and (2) the alternating layers nTM&lt;nM&lt;nTE, and the indices of the upper cladding and the lower cladding are less than nTM, nM and nTE. A polarizer, polarizing beam splitter and coupler using clipped coupling can employ the material having an index nM and the alternating layers.

CROSS REFERENCE

This application is a continuation-in-part (CIP) application of PCTapplication number PCT/US2017/036505 entitled “PHOTONIC APPARATUS,METHOD, AND APPLICATIONS” filed Jun. 8, 2017 which claims the benefit ofU.S. Provisional Patent Application Ser. No. 62/347,212 filed Jun. 8,2016, the subject matter of both being herein incorporated by referencein their entireties.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Career Award #ECCS1150672 awarded by The National Science Foundation. The government hascertain rights in this invention.

FIELD

This Disclosure relates to photonic methods and apparatus forcontrolling polarization, and in particular integrated photonic methodsand apparatus for controlling polarization.

BACKGROUND

Polarization management is generally a critical requirement forstate-of-the-art integrated photonic systems. Conventional photonicstructures exhibit a high degree of asymmetry in the vertical direction,either by design or by limitations of the particular fabrication methodemployed. Also, the achievable refractive index contrast, which affectsthe achievable compactness of the photonic structures desired, istypically small, which has the particular disadvantage of poortemperature stability, and cannot be brought to very small dimensions asa restriction of methods employed. Key building blocks such aspolarizers and polarization beam-splitters (PBS) to date have achievedoperation over only limited optical bandwidths, thus limiting theiruses.

Polarization management in modem integrated photonics is conducted in avariety of ways, depending on the processes available or the platformsconsidered. Most commonly, a polarization-filtering effect is achievedusing a metal cladding or grating on the surface of a waveguide, whichintroduces large losses for one polarization but not for the otherpolarization. Alternatively, shallow etching is applied to one area of awaveguide such that the transverse-magnetic (TM) light will leak out.These methods generally require significant additional processing on awafer in order to achieve polarization. Furthermore, if both TM andtransverse-electric (TE) polarization are desired, the amount ofadditional processing increases further since separate designs areneeded to process TE polarized light and the TM polarized light.Additionally, large losses for the “pass” polarization can result fromtheir interaction with metal claddings, or from transitions betweenshallow and deep etched regions. The demonstrated bandwidths ofconventional integrated polarizes and PBS are fairly limited, generallynot exceeding 100 nm in the telecom band.

Concerning integrated PBS devices, state-of-the-art approaches aretypically precision-engineered directional couplers that selectivelycouple one polarization into a specific output channel but not the otherpolarization. Although they can be quite compact, they generally eitherrequire difficult-to-fabricate geometries (e.g., two waveguides ofdifferent height next to each other), or complicated additionalprocessing steps. Still, the bandwidths are typically limited to <300 nmeven in simulated designs.

In the telecommunications market, polarization diversity functions areoften implemented in fiberized components that are bulky and expensive.In remote optical sensing, it is often desirable to extract informationon the polarization dependence of a received signal from a target.Additionally, spectroscopic analysis may be needed simultaneously withthe information extraction. Such processing can generally be achievedwith free-space optics, but such free-space optics systems are alignmentsensitive and expensive to implement.

SUMMARY

This Summary is provided to introduce a brief selection of disclosedconcepts in a simplified form that are further described below in theDetailed Description including the drawings provided. This Summary isnot intended to limit the claimed subject matter's scope. Aspects of aninvention are defined in the appended independent claims.

This Disclosure recognizes there is a need for improved integratedphotonic devices to control polarization. Disclosed photonic devices forcontrolling polarization utilize a fundamentally different techniquewhere a birefringent effect is derived from the behavior of a multilayerfilm having an alternating refractive index (alternating layers of afirst material having a first index of refraction and second materialhaving a second index of refraction) in the device layer itself, withthe alternating layers in the device layer being in the core film or theside cladding film around the core film. Disclosed photonic devices arefor performing polarization-selective operations on integrated photonicchannels, that enables greater optical bandwidths (e.g., more than anoctave in frequency) to be achieved in devices such as polarizers andPBS's. Furthermore, disclosed techniques enable optical functionalitythat is not possible with conventional photonic devices. For example,disclosed photonic devices for controlling polarization enable theco-existence of “transverse-electric-only” and“transverse-magnetic-only” single-polarization waveguides on a samelayer of a single device, as well as waveguides supporting bothpolarizations. Disclosed configurations lead to other novel devices suchas photonic resonators that are nearly invisible to light of onepolarization, but which interact normally with light polarized in theorthogonal direction.

Some disclosed aspects enable both TE and TM polarizers to beimplemented together, and with no additional processing compared to thenormal fabrication flow. Furthermore, in some aspects, because of thehigh degree of symmetry in the structure and its wavelength-independentoperation, bandwidths spanning an octave are achievable, representing alandmark improvement over conventional devices.

Disclosed aspects include an arrangement of optical materials on asubstrate (a photonic device) that enables precise and spatiallyvariable control of the refractive index experienced by light ofdifferent polarizations interacting with the photonic device. As such,optically anisotropic features can be exploited for useful functions onan integrated photonic platform.

In accordance with one aspect, there is provided a photonic device forguiding light in a first direction, the light having a wavelength λ, thedevice having a polarization-dependent region comprising a lowercladding layer, and a device layer disposed on the lower cladding layer.The device layer comprises a first cladding film and a second claddingfilm, and a core film extending in the first direction between the firstcladding film and the second cladding film. The device further comprisesan upper cladding layer disposed on the device layer. The core filmcomprises one of (1) a material having an index of refraction n_(M) and(2) alternating layers of a first material having a first index ofrefraction and second material having a second index of refractiondifferent than the first material, the alternating layers having aneffective index of refraction for TE polarized light n_(TE) and aneffective index of refraction for TM polarized light n_(TM). Each of thefirst cladding film and the second cladding film comprises the other of(1) the material having the index of refraction n_(M) and (2) thealternating layers. n_(TM) is greater than n_(M) which is greater thann_(TE) at the wavelength λ. The index of refraction of the uppercladding and the index of refraction of lower cladding are both lessthan n_(TM), n_(M) and n_(TE) at wavelength λ.

In some embodiments, the difference between n_(TM) and n_(M) issubstantially equal to the difference between n_(M) and n_(TE), at thewavelength λ. The difference between n_(TM) and n_(TE) may be in therange 0.01 to 0.8.

In some embodiments the core film comprises the material having an indexof refraction n_(M). In some embodiments, the core film comprises thealternating layers. In some embodiments, the device further comprises aninput electromagnetic (E/M) waveguide having an input core opticallycoupled to the core film.

The input E/M waveguide may comprise an input E/M waveguide lowercladding layer, an input E/M waveguide device layer disposed on theinput E/M waveguide lower cladding layer, the input E/M waveguide devicelayer comprising an input E/M waveguide first cladding film and an inputE/M waveguide second cladding film, and the input core extending in thefirst direction between the input E/M waveguide first cladding film andthe input E/M waveguide second cladding film, and an input E/M waveguideupper cladding layer disposed on the input E/M waveguide device layer.The input core comprises a material having index of refraction greaterthan the index of refraction of the input E/M waveguide lower claddinglayer, the input E/M waveguide first cladding film, the input E/Mwaveguide second cladding film and the upper cladding layer atwavelength λ.

In some embodiments, the device is a polarizer, the device furthercomprising a first transition region disposed between the inputwaveguide and the polarization-dependent region where a width of thefirst cladding film is disposed between the core film and a width of theinput E/M waveguide first cladding film, and a width of the secondcladding film is disposed between the core film and a width of the inputE/M waveguide second cladding film. In the first transition region, (1)the width of the first cladding film increases and the width of theinput E/M waveguide first cladding film decreases along the firstdirection and (2) the width of the second cladding film increases andthe width of the input E/M waveguide second cladding film decreasesalong the first direction.

In some embodiments, the device further comprising an output E/Mwaveguide having an output core optically coupled to the core film, atan opposite end from the input E/M waveguide.

In some embodiments, the output E/M waveguide comprises an output E/Mwaveguide lower cladding layer, an output E/M waveguide device layerdisposed on the output E/M waveguide lower cladding layer, the outputE/M waveguide device layer comprising an output E/M waveguide firstcladding film and an output E/M waveguide second cladding film, and theinput core extending in the first direction between the output E/Mwaveguide first cladding film and the output E/M waveguide secondcladding film, and the E/M waveguide comprises an output E/M waveguideupper cladding layer disposed on the output E/M waveguide device layer.The output core comprises a material having an index of refractiongreater than the index of refraction of the output E/M waveguide lowercladding layer, the output E/M waveguide first cladding film, the outputE/M waveguide second cladding film and the upper cladding layer atwavelength λ.

In some embodiments, the device further comprise a second transitionregion disposed between the output waveguide and thepolarization-dependent region where a width of the first cladding filmis disposed between the core film and a width of the output E/Mwaveguide first cladding film, and a width of the second cladding filmis disposed between the core film and a width of the output E/Mwaveguide second cladding film. In the second transition region, (1) thewidth of the first cladding film decreases and the width of the outputE/M waveguide first cladding film increases along the first directionand (2) the width of the second cladding film decreases and the width ofthe E/M waveguide second cladding film increases along the firstdirection.

In some embodiments, the polarizer is a TE-pass polarizer. In someembodiments, the polarizer is a TM-pass polarizer.

Another disclosed aspect is directed to a photonic device to guide lightin a first direction, the light having a wavelength λ, comprising: a buswaveguide; and a second waveguide having a core characterized by a widthequal to W transverse to the core and the bus waveguide, the secondwaveguide having a first tapered region proximate the bus waveguide inwhich the width is reduced along the first direction, and a secondtapered region proximate the bus waveguide in which the width isincreased along the first direction back to W, the second waveguidebeing evanescently coupled to the bus waveguide between the firsttapered region and the second tapered region. The device furthercomprises a cladding material disposed between the bus waveguide and thesecond waveguide, and the second waveguide comprising one of (1) amaterial having an index of refraction n_(M) and (2) alternating layersof a first material having a first index of refraction and secondmaterial having a second index of refraction different than the firstmaterial, the alternating layers having an effective index of refractionfor TE polarized light n_(TE) and an effective index of refraction forTM polarized light n_(T)v. The bus waveguide comprises the other of (1)the material having an index of refraction n_(M) and (2) the alternatinglayers, and n_(TM)<n_(M)<n_(TE) at the wavelength λ, and the index ofrefraction of the upper cladding and the lower cladding is less thann_(TE), n_(TM) and n_(TE) at wavelength λ.

In some embodiments, the second waveguide is a ring waveguide. In someembodiments, the bus waveguide comprises the material having an index ofrefraction n_(M). In some embodiments, the bus waveguide comprises thealternating layers. Still further aspects of the invention are directedto a photonic device to guide light in a first direction and to dividethe light into a first output having only TE-polarized light and asecond output having only TM-polarized light, the light having awavelength λ, the device comprising a lower cladding layer, a devicelayer disposed on the lower cladding layer, an upper cladding layerdisposed on the device layer, the device layer comprising a firstcladding film and a second cladding film, and a core film extending inthe first direction between the first cladding film and the secondcladding film, the core film comprising a transition region and aseparation region. In the transition region, the core film comprises aninput core having a first width transverse to the first direction and atransition core contacting the input core and the transition core havinga second width that increases along the first direction until core filmhas a width equal to the 1.3 to 3.0 times the input core width; and inthe separation region, the input core is separated from the transitioncore by a separation distance that increases along the first directionto a size that prevents coupling of the light of wavelength λ betweenthe input core and the transition core.

The input core comprises one of (1) a material having an index ofrefraction n_(M) and (2) alternating layers of a first material having afirst index of refraction and second material having a second index ofrefraction different than the first material, the alternating layershaving an effective index of refraction for TE polarized light n_(TE)and an effective index of refraction for TM polarized light n_(TM). Thetransition core comprises the other of (1) the material having an indexof refraction n_(M) and (2) the alternating layers. n_(TE) is less thann_(M) which is less than n_(TE) at the wavelength λ. Each of the uppercladding layer, the lower cladding layer, the first cladding film, thesecond cladding film and the separation cladding film have an index ofrefraction less than each of n_(TM), n_(M), n_(TE) at wavelength λ.Accordingly, the input core forms the output of only a first ofTE-polarized light and the TM-polarized light, and the transition coreforms an output of only a second of the TE-polarized light and theTM-polarized light. The input core and the transition core are separatedfrom one another in the separation region at an angle from 0.1 to 10degrees.

In some embodiments, the first width equals the second width where thetransition region and the separation region meet.

In some embodiments, the device further comprises a second transitionregion where the transition core has a constant width and the input corehas a width that increases along the first direction.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made to the accompanying drawings, which are notnecessarily drawn to scale, wherein:

FIG. 1 is a cross-sectional view of an example of a photonic device forpropagating only light having a TM-polarization, according to aspects ofthe present invention;

FIG. 2 is an expanded cross-sectional view of Film C of the photonicdevice of FIG. 1;

FIG. 3 is a cross-sectional view of an example of a photonic device forpropagating only light having a TE-polarization, according to aspects ofthe present invention;

FIGS. 4A and 4B are cross-sectional views of examples of photonicdevices for propagating both TE-polarized and TM-polarized light,according to aspects of the present invention;

FIGS. 5A-5C are cross-sectional views of examples of transition regionsof photonic device for controlling polarization, according to aspects ofthe present invention;

FIG. 6 is a top-view of an example of an arrangement of materials of aTM-pass polarizer (having an upper cladding omitted to facilitateviewing) according to aspects of the present invention;

FIGS. 7A and 7B are top view snapshots of the propagation of TM and TElight, respectively, of a two-dimensional simulation of a polarizer asdescribed with reference to FIG. 6;

FIG. 8 is a top-view of an arrangement of materials of an example of aTE-pass polarizer according to aspects of the present invention;

FIG. 9 is a top-view of an example of an arrangement of materials of aPBS according to aspects of the present invention.

FIGS. 10A and 10B are 2D top view simulations of a PBS having thearrangement of FIG. 9 showing the TM-polarized and TE-polarized light,respectively;

FIG. 11 is a top view of another embodiment of a PBS according toaspects of the present invention;

FIG. 12 is a top view schematic of a TE-cloaked resonator having aclipped resonator waveguide;

FIGS. 13A and 13B are top views of two-dimensional simulations of theTE-clocked resonator of FIG. 12 showing a portion of the TM-polarizedlight being directed into the resonator, and negligible TE-polarizedlight being directed into the resonator, respectively;

FIG. 14 is a top view schematic of a TM-cloaked resonator achieved byreversing the core materials of the bus and resonator of FIG. 12;

FIG. 15 is a top view of a direction coupler employing “clipped”coupling for coupling-out a fraction of energy of TE-polarized lightfrom bus waveguide to second waveguide;

FIGS. 16-26 illustrate selected steps of methods of fabricating devicesaccording to some aspects of the present invention;

FIG. 27 illustrates an example of an embodiment of a system according toaspects of the present invention including a wave guide device added toaugment a pre-processed substrate;

FIGS. 28A and 28B show low-loss propagation as observed from the topsurface of a chip having a TM-only waveguide and a TE-only waveguide,respectively; and

FIG. 29 includes top and bottom scanned digital optical images on theright side being an image of TE-polarized light and TM-polarized lightexiting the ports of a PBS, and on the left side corresponding toreference paths.

DETAILED DESCRIPTION

Example aspects are described with reference to the drawings, whereinlike reference numerals are used to designate similar or equivalentelements. Illustrated ordering of acts or events should not beconsidered as limiting, as some acts or events may occur in differentorder and/or concurrently with other acts or events. Furthermore, someillustrated acts or events may not be required to implement amethodology in accordance with this disclosure.

FIG. 1 is a cross-sectional view of an example of a photonic device 100according to a disclosed aspect configured to guide light in aZ-direction, the light having a wavelength λ. As used herein, theZ-direction is shown as the light propagation direction including asshown in FIG. 1 (being the direction into the page), and the X-Y planeis the cross-sectional plane that is perpendicular to the Z-direction.When herein referring to the in-plane direction, with this convention,this refers to the X-Z plane. Photonic device 100 comprises opticalmaterials that can be deposited onto a thick handle substrate(substrate) 110 (e.g., a wafer, generally several 100s of μms thick) toform the features enumerated below. With this convention, the substrate110 has an in-plane oriented area dimension along the X-Z plane and athickness dimension in the Y-direction.

Photonic device 100 comprises a lower cladding layer 120 shown as FilmA, a device layer 130 comprising a first side cladding film 132 a shownas Film C and a second side cladding film 132 b also shown as Film C, acore film 134 shown as Film B, and an upper cladding layer 140 shown asFilm D. The device layer 130 as described in detail below has a periodicstructure with an orientation where the periodic films of the devicelayer 130 are oriented parallel to the in-plane direction of thesubstrate 110 is substantially different conventional verticallyoriented (stacked) layers which are oriented in the thickness dimensionof the substrate 110 (and perpendicular to the in-plane dimension of thesubstrate). Disclosed arrangements have the advantage of precisioncontrol in thicknesses via standard deposition methods, as well asextremely low interface roughness, in contrast to conventionalvertically oriented elements.

Examples of methods for fabricating structures such as photonic device100 are provided below with reference to FIGS. 16 to 26 that aredescribed below. The cross-sectional structure of the photonic device100 in FIG. 1 remains the same in the Z-direction Z (the (the lightpropagation direction) as described, where the Z-direction is along theX-Z plane (the in-plane direction of the substrate 110). over a selectedlength; however, as discussed below with reference to FIGs describedbelow, a device having a cross section as illustrated in FIG. 1 can becombined with one or more structures to form devices having a differentcross section, to selectively process light propagating in theZ-direction.

The substrate 110 can be constructed of any suitable material currentlyknown or later developed for maintaining photonic devices describedherein. For example, the substrate 110 may have a thickness between 300and 1,000 μm. The lower-cladding film 120 is formed on the top surfaceof substrate 110 and has a refractive index n_(A). For example, thelower-cladding film 120 may have a thickness between 0.5 to 10 μm.

The device layer 130 is formed on top of the lower cladding film 120.Device layer 130 comprises first side cladding film 132 a and secondside cladding film 132 b, and the core film 134 extends in theZ-direction between the first side cladding film 132 a and second sidecladding film 132 b. In the illustrated embodiment, the core film 134has a refractive index n_(m) and the film for the first side claddingfilm 132 a and second side cladding film 132 b is anisotropic. It istypically desirable that the films of the device layer 130 (e.g., corefilm 134 and film for the side claddings have thicknesses equal to oneanother (ideally within a difference of less than 10% between the heightof core film 134 and the height of film for the side claddings. Asillustrated in FIG. 2, the film for the side claddings shown as Film Cis constructed of alternating layers of two different materials shown aslayer 210 and layer 220 that alternate (are periodic) in the X-Z planeas shown which have refractive indices n_(H) and n_(L), respectively.The index of refraction n_(H) of layer 210 is higher than the index ofrefraction of layer 220, at working wavelength λ. Any suitable lightsource (not shown) may be present to provide the light of wavelength λ.For example the light source may generate the light such as a laser, orthe light source may direct the light to the device such as an opticalfiber or another waveguide (e.g., an E/M waveguide as described below).

The alternating layers 210, 220 have thicknesses that are much smallerthan the shortest optical wavelength λ that is to be processed byphotonic device 100 such that the effective medium approximation holds.For example, the thicknesses of the alternating layers 210, 220 in thecross-sectional (XY) plane may be less than 1/10th the effectivewavelength of the light inside the core material. As a result of therelatively small thicknesses of layers 210, 220, the film for the sidecladdings can be considered to have an effective refractive index thatdepends on the relative thickness and index of each particular layer,and which also depends on the polarization of light under considerationdue to the different continuity relations for the electric and magneticfields. Mathematical expressions of the effective medium approximationare shown below as Equations 1(a) and 1(b) for TE and TM light,respectively. Layers 210 may be identical to one another, and layers 220may be identical to one another; however, some variation is possible.

The effective refractive indices for the transverse-electric (TE) andtransverse-magnetic (TM) polarizations, n_(TE) and n_(TM)T, are chosento be greater than and lower than the refractive index n_(M) of the corefilm 134, (n_(TM)<n_(M)<n_(TE)), respectively. It is typically desirablethat indices n_(TE) and n_(M) are separated from refractive index n_(M)by an approximately equal value. A relatively larger separation betweenn_(TM) to n_(TE) is generally desirable; for example a separation of0.01 to 0.8 at operating wavelengths λ is generally advantageous. Insome embodiments the separation is in the range 0.1 to 0.8 at anoperating wavelength.

Upper cladding 140 is disposed on top of the device layer 130, andtypically has a thickness between 0.5 to 10 μm, comprising a materialwith a refractive index n_(D) that is approximately equal to that of thelower cladding 120, n_(A). Refractive index n_(D) advantageously iswithin a difference of <0.05 refractive index units of n_(A). Also, theindex of refraction of the upper cladding 140 and the lower cladding 120is less than n_(M) (i.e., the refractive index of the core film) atwavelength λ, and the index of refraction of the upper cladding 140 andthe lower cladding 120 is typically less than n_(TM) and n_(TE) atwavelength λ.

It will be appreciated that when the above arrangement is realized, itbecomes possible to propagate light in core film 134 that exhibitsdifferent behaviors depending on the polarization of light in use. Forexample, in the embodiment of FIG. 1, the device layer 130 is shown tocomprise mainly of the film for the side cladding films 132 a, 132 b,except for a narrow rectangular core film 134 (corresponding to arectangular waveguide extending along the Z-direction), which comprisesanother film shown as Film B. For light that is polarized in thevertical direction (the Y-direction), corresponding to the TM opticalmode, the effective refractive index of the surrounding side claddingfilms 132 a, 132 b is lower than the refractive index of the core film134, causing the optical mode to be confined to core film 134 throughtotal internal reflection. However, for light that is polarized in thehorizontal direction (along the X-direction), corresponding to the TEmode, the effective refractive index of the film surrounding sidecladding 132 a, 132 b is greater than that of the core film 134. Thus,TE-polarized light is not confined to core film 134, and is radiated outof core film 134. In other words, the core film 134 does not support amode for the TE polarization, although core film 134 does support a modefor TM-polarized light. Photonic device 100 can thus be considered a“TM-only” waveguide. Such polarization-dependent operation holds for allwavelengths λ for which the effective medium approximation is satisfiedmeaning that there is a short-wavelength cutoff but there is no cutoffin the long-wavelength regime, and for which the relationshipn_(TM)<n_(M)<n_(TE) is maintained.

It can also be recognized that the same approach allows for constructionof “TE-only” waveguides. An example of a TE-only photonic device 300 isillustrated in FIG. 3, which is the “complement” of the arrangementshown in FIG. 1. Photonic device 300 is the result of switching thematerial of core 334 and the cladding 332 a, 332 b materials relative tothose of the TM-only arrangement of device 100. Accordingly, in photonicdevice 300, because the refractive index of the cladding 332 a, 332 b islower than that of the core film 334, TM-polarized light is no longerguided, but the TE-polarized light is. Thus disclosed techniques readilyenable broadband “TM-only” and/or “TE-only” single-polarizationwaveguides to be realized on a common layer, without any additionalprocessing beyond normal fabrication flow required to define each type.Also, the index of refraction of the upper cladding 140 and the lowercladding 120 is less than n_(TM) and n_(TE) (i.e., the indices of thecore) at wavelength λ, and the index of refraction of the upper cladding140 and the lower cladding 120 is typically less than n_(M) atwavelength λ.

It will be appreciated that in each of photonic devices 100 and 300, thecore films 134, 334 comprise one of (1) a material having an index ofrefraction n_(M), and (2) alternating layers of a first material havinga first index of refraction n_(H) and second material having a secondindex of refraction n_(L) different than the first material. Thealternating layers have an effective index of refraction forTE-polarized light n_(TE) and an effective index of refraction forTM-polarized light n_(TM), where n_(TE) and n_(TM) are defined asfollows.

(n _(TE))² =f×(n _(H))²+(1−f)(n _(L))²  Equation 1(a)

1/(n _(TM))² =f/(n _(H))²+(1−f)/(n _(L))²  Equation 1(b)

where n_(TE)<n_(M)<n_(TE) at the wavelength λ, and

f is the fill factor of the n_(H) material in an n_(H)−n_(L) pair oflayers.

Each of first cladding film 132 a, 332 a and the second cladding film132 b, 332 b comprise the other of (1) the material having an index ofrefraction n_(M) and (2) the alternating layers.

It will be appreciated that embodiments of systems employingpolarization-dependent devices as described above, also support theintegration of more conventional waveguides (i.e., that supportpropagation of both polarizations TE and TM) onto the device layer. Itwill be appreciated that such ability is desirable since for manyapplications, such as high-speed communication or remote sensing, it isdesirable to transmit and subsequently process both polarization statesin order to preserve flexibility of design. For example, conventionalwaveguides can be achieved by arranging the materials (of photonicdevices 100 and 300) as in FIG. 4A or FIG. 4B.

FIGS. 4A and 4B show a waveguide core 434, 434′ comprising Film B andFilm C, respectively, with the cores being surrounded by a cladding of aFilm D 440 on the top and sides 432 a and 432 b and a bottom Film A 420on the bottom. In these arrangements, the refractive indices n_(A) andn_(D) are sufficiently low in comparison to the birefringent indicesn_(TE) and n_(TM) of Film B and C such that modes of both of thepolarizations are supported. The horizontal extent of Film D 440 needonly be large enough to prevent evanescent coupling from the waveguidecore into the rest of the device layer. For example, typical minimumwidths may be from 0.5 to 3 microns. Waveguides capable of supportingpropagation both polarizations will be referred herein as E/Mwaveguides.

It is to be appreciated that combining the arrangements discussed withreference to FIGS. 1, 3, 4A and 4B enables layout and usage of uniquewaveguide devices that can achieve any desired polarization state, frompure TE operation, to bi-polarized operation, to pure TM operation. Toachieve polarization-selective photonic devices using this technology,transitions between waveguides of these types may be exploited, whichdivide light based on polarization. During transitions to and from anyof the arrangements of FIGS. 1, 3, 4A and 4B, a hybrid arrangement ofmaterials may be employed, such as those in FIGS. 5A-5C. In all threehybrid arrangements, the relative widths of the Film B and Film C in thecore region 534 are varied along the Z-direction and may take on anyvalue in-between initial and final widths corresponding to otherarrangements as described herein. These particular transition regionswill be referred to in the context of examples of polarization-selectivedevices achievable using techniques according to aspects of the presentinvention. Typically, transition regions include a lower cladding 520,side claddings 532 a and 532 b, an upper cladding 540, all on asubstrate 510.

One such device is a polarizer. Functionally, a polarizer is a devicethat is inserted into an optical waveguide path, which effectivelyattenuates light of one polarization but leaves the other polarizationunaffected. It will be appreciated that, in a polarizer device, an uppercladding and lower cladding are present, but omitted from the FIG. 6 forease of description. FIG. 6 is a top-view of a device layer of anexample of a polarizer according to aspect of the present invention. Forexample, a TE-blocking polarizer 600 could be implemented as follows.Light is received from an input 602, which is an E/M waveguide (e.g.,having a width 300 to 3000 nm) which undergoes an adiabatictransformation to a TM-only waveguide 606 by means of tapering away theFilm D cladding and replacing it with Film C in an area surrounding thecore, over a characteristic length (e.g., between 5 to 100 microns) in atransition region 604.

The transition region is sufficiently long so as to prevent couplinginto higher-order modes. It will be appreciated that the arrangementcorresponds to that seen in FIG. 5B. During propagation along thetransition region, the TE light begins to radiate away as it does notbelong to a supported mode in the waveguide. After the transition iscomplete (at which point the arrangement corresponds to FIG. 1), aTM-only waveguide is formed which propagates for some distance (e.g.,between 10 to 300 microns) to ensure that no stray TE-polarized lightremains, and then it undergoes a reciprocal transformation back (intransition region 608) to the E/M waveguide arrangement before reachingan output region 603. thus TM light is unaffected, while TE light iscompletely removed from the path, representing ideal polarizingbehavior. It will be appreciated that propagation in regions 604, 606and 608 is polarization-dependent, and regions 604, 606 and 608,individually and together, form a polarization-dependent region ofpolarizer 600. It will also be appreciated that although the abovedescription assumed all light of a selected polarization was removed, indevices according to aspects of the present invention, the light ofselected polarization may be removed to any suitable degree.

A snapshot of a two-dimensional simulation of a polarizer as describedwith reference to FIG. 6 is provided in FIGS. 7A and 7B. The simulateddevice showed <0.1 dB loss for the TM polarization and 23 dB attenuationfor the TE polarization at a wavelength of 633 nm. When the wavelengthis doubled (1266 nm), the loss for the TM polarization was 0.05 dB andthe attenuation for the TE polarization was 31 dB. To date, it isbelieved no simulated or fabricated integrated polarizer has everachieved such a wide bandwidth. It will be appreciated that higherextinction ratios can be achieved trivially by increasing the length ofthe single-polarization section. For example, simulations can beperformed using the MULTIPHYSICS® simulation software available fromCOMSOL®, Inc of Burlington, Mass.

Polarizers that block TM light are also readily achieved usingtechniques according to aspects of the present invention. FIG. 8 is atop-view of a device layer of an appropriate arrangement of material ofan example of a polarizer 800 according to aspects of the presentinvention. An input E/M waveguide 802 and output E/M waveguide 803utilize Film B as the core. The core is replaced by Film C at the onsetof the initial taper of transition region 804 and likewise at the end ofthe taper of transition region 808 at output 803. It will be appreciatedthat propagation in regions 804, 806 and 808 is polarization-dependent,and regions 804, 806 and 808, individually and together, form apolarization-dependent region of polarizer 800.

Another integrated photonics device according to disclosed aspects is aPBS. A PBS is capable of taking the common input 904 (i.e., an input ofTE-polarized and TM-polarized light) and splitting light of eachpolarization into a separate output 906, 908 as shown in FIG. 9. It istypically desirable that the splitting occur with low intrinsic lossesand low crosstalk (the undesired polarization in a given output).

FIG. 9 is a top-view of an example of an arrangement of materials of aPBS 900 according to a disclosed aspect. The input waveguide is an E/Mwaveguide 902 with core 934 consisting of Film B and side claddings 932a and 932 b of film D, with the waveguide core 934 having a width of300-3000 nm and a thickness of 100 nm to 2,000 nm. Structurally, PBS 900is similar to a conventional “Y-junction splitter,” a component thatacts as a 50:50 splitter. Core 934 is gradually widened by a factorbetween 1.3 to 3 times over a transition region 907 (e.g., having alength between 5 to 100 microns). In the illustrated embodiment, core934 comprises an input core 936 and a transition core 938. In theillustrated embodiment, in transition region 907, the input core 936 hasa uniform width and the transition core 938 has a width that increasesalong the direction of propagation of light. In the illustratedembodiment, core 934 is widened on its lower side as transition core 938gets wider. In the illustrated embodiment the transition core consistsof Film C.

In the illustrated embodiment, once the transition core reaches itsmaximum width, the waveguide core consists of equal parts of Film B andFilm C. In the transition region 907, the core has a hybrid arrangementcomprising film B and film C. Next, in the separation region 909, thetwo core materials (i.e., Film B and Film C) are split apart by atriangular wedge as the Film D cladding is introduced between corematerials. For example, the internal angle at which the two cores aresplit may be from 0.1 to 10 degrees. The above arrangement results inthe TM and TE polarizations splitting into separate arms with a highdegree of efficiency. Once the arms are diverged by a sufficient spacingso as to prevent coupling between the arms, the lower arm correspondingto where the TE output is diverted may be replaced with Film B again(not shown). The upper arm with its Film B core is where the TM outputlight is diverted.

At 633 nm wavelength, an insertion loss of 0.16 dB was calculated forthe TE output, and 0.05 dB for the TM output. For a 1266 nm wavelength,the loss for both ports becomes negligible (<0.01 dB). FIGS. 10A and 10Bare snapshots of splitting simulations for the TM-polarized andTE-polarized 1266 nm light, respectively, for a PBS 900 shown in FIG. 9.It is apparent that the TM light is diverted into the upper waveguideand the TE light is directed into the lower waveguide.

FIG. 11 is a top view of another embodiment of a PBS 1100 according toaspects of the present invention. An input E/M waveguide 1102 having acore 1134 (Film B) providing an input into PBS 1100. Core 1134 iswidened in a first transition region 1104 using Film C. In a secondtransition region 1106, core 1134 is widened using Film B, while keepingthe width of the Film C fixed. Multiple spatial modes may be supportedin the waveguides in this particular embodiment, although the length ofthe structure can be made sufficiently long to avoid coupling into thosemodes; a total PBS length of 30 to 500 microns may be suitable dependingon the wavelength of interest. The maximum width of each core should belarge enough such that the optical modes for TE and TM polarizations arewell-separated once the two core materials are split apart in separationregion 1108 by a wedge of Film D. A suitable maximum width could be from800 to 3000 nm for each core area, and each widening section could bebetween 10-100 microns in length. Although in the embodiments describedabove the input cores were made of Film B and the transition cores weremade of Film C, it will be appreciated that an input core can be made ofFilm C and that a transition core can be made of Film B.

Techniques according to aspects of the preset invention also enableoptical devices that are not possible with conventional integratedphotonics. The ability to design anisotropy into specific structures canbe used to change coupling conditions between different waveguides. Oneexample apparatus/application of this is a “polarization-cloakedresonator”. A polarization-cloaked resonator consists of a circular ringwaveguide (ring resonator) coupled to a “bus” waveguide. The nominalwidths of both waveguides are ideally chosen to confine only a singletransverse optical mode in the horizontal (in the plane of the surface)direction.

FIG. 12 is a top view schematic of a TE-cloaked resonator 1200. In oneembodiment of a “TE-cloaked resonator,” the bus waveguide core 1220consists of film C. The ring waveguide core 1210 is of film B having awidth W transverse to core 1210. Both waveguides are of the E/M type,having a surrounding cladding 1230 consisting of Film D. The buswaveguide passes within a small gap G (100 nm to 3,000 nm) away from thering waveguide to control the amount of light that is evanescentlycoupled in and out of the resonator 1200. In the vicinity of theirsmallest separation, a tapering or “clipping” of the ring waveguide isapplied to gradually reduce its width (in a direction transverse to thecore longitudinal axis) in region C₁, and then in a region C₂ the width(transverse to the core longitudinal axis) is increased it as it passesaway from bus 1220. Light travels in direction Z. It will be appreciatedthat region C₂ is further along direction Z than region C₁. The extentof clipping should consist of a reduction in the ring waveguide widthfrom W, by a fraction between 0.1 and 0.6. The clipping, combined withthe intrinsic anisotropy due to film C in the bus waveguide results inonly TM-polarized light being coupled into ring resonator 1210. ForTE-polarized light injected through the input, almost none is coupledinto the resonator and no losses result from it. Simulation results areprovided in FIG. 13 for light having a wavelength 1000 nm show that theTM power coupling can be 38 dB stronger than the TE power coupling.

FIG. 14 is a top view schematic of a TM-cloaked resonator 1400 achievedby reversing the core materials of the bus waveguide 1420 and ringwaveguide 1410 relative to the bus waveguide 1220 and ring waveguide1210 of resonator 1200. In resonator 1400, the opposite relationshipbetween polarizations is achieved relative to resonator 1200. It is tobe appreciated that the polarization-selective nature of this couplingstrength is maintained over a broad wavelength range since it does notdepend on resonant effects. It is also to be appreciated that the same“clipped” coupling approach is useful for directional couplers, whichare designed to transmit a fraction of power from one waveguide toanother.

FIG. 15 is a top view of a direction coupler 1500 employing “clipped”coupling for coupling-out a fraction of energy of TE-polarized lightfrom bus waveguide 1520 to second waveguide 1510, but not theTM-polarization light. It will be appreciated that film B and Film Cmaterials of the bus waveguide and second waveguide may be swapped tocouple the orthogonal polarization of light to couple-out a fraction ofenergy of TM-polarized light, but not TE-polarized light.

Example methods of fabricating the above-described devices and materialarrangements are described below. It will be appreciated that thedevices and material arrangements described above are not limited tothose constructed using methods described.

Selected steps of methods of fabricating are described below withreference to FIGS. 16-26.

1. In FIG. 16, a substrate/handle wafer is coated with Film A (lowercladding), followed by depositing a Film C on Film A. As describedbelow, Film C after etching will form a part of the side cladding aroundthe later formed core film (Film B) to provide one of the alternatinglayers of a first material having a first index of refraction and secondmaterial having a second index of refraction that is different than thefirst material.

2. In FIG. 17, a suitable etch mask such as comprising photoresist iscoated and patterned with lithography to facilitate formation offeatures to be etched into Film C to enable forming the core film (filmB) in the trenches between Film C features;

3. In FIG. 18, selective dry etching of Film C is performed all the wayto the top interface of Film A. Although only 1 trench through Film C isshown for simplicity, there is a plurality of trenches simultaneouslyformed (typically at least hundreds of trenches formed, for examplethousands of trenches, to enable forming a disclosed alternatingstructure.

4. In FIG. 19, the resist etch mask shown as resist is removed usingsolvent cleaning or plasma cleaning;

5. In FIG. 20, Film B (the core film) is deposited onto the wafersurface, to a sufficient thickness so that any etched surfaces (thetrenches between the Film C features) are completely filled and aregenerally provided to a minimum height of 300 nm above the top surfaceof Film C;

6. In a next step, excess material of Film B is removed to flatten theoverall surface of the wafer at the level of the top surface of Film C(commonly referred to as planarization). Planarization may be achieved,for example, using one of the following techniques:

-   -   (i) In FIG. 21A, the surface of Film B is coated with a polymer        film with suitable planarization characteristics, followed by        plasma etching with equal selectivity between the resist and        Film B (as illustrated in FIG. 21B) until the top surface of        Film C is reached, or    -   (ii) In FIG. 22, Film B chemical-mechanical polished (CMP) is        used to bring it down to the top surface of Film C and thereby        flatten it;

7. In FIG. 23, an additional lithography is performed to pattern an etchmask shown as resist into some areas of the wafer surface, shown withresist openings over Film B, wherever prescribed by the necessarydesign;

8. In FIG. 24, exposed areas of Film B are plasma etched to formtrenches down to the top interface of Film A;

9. In FIG. 25, the resist is removed generally using a solvent or plasmacleaning;

10. In FIG. 26, a 0.5 μm to 5 μm thick layer of Film D (the uppercladding) is deposited to cover the etched sidewalls of any features(trenches) in Film B, and to fully cover the top surface, making theresulting photonic device structure completed in the vertical direction.As a result, in this example the side cladding comprises film D aroundthe core film B, with the alternating structure comprising film Cfeatures alternating with film D features.

The method of fabrication described above can be applied using any of aplurality of materials as films A-D, as well as the handle/substrate.Films A-D may comprise either dielectric or semiconductor materials, orsome combination thereof. Dielectric materials, can include (but are notlimited to) silicon-based compounds such as amorphous silicon, silicondioxide, silicon nitride, silicon oxynitride, silicon carbide or siliconmonoxide. Other materials of interest can include tantalum pentoxide,titanium dioxide, zinc sulfide, zinc selenide, hafnium oxide, aluminumoxide, aluminum nitride, silicate compounds (including glasses such asHYDEX), or fluoride compounds such as magnesium fluoride or calciumfluoride. In principle, any dielectric materials may be used for filmsA-D, provided their combination satisfies the refractive indexrelationships as set forth above. Chalcogenide materials may be employedas well given their large tunability in refractive index; such materialsmay include variable glass compositions employing germanium, arsenic,sulfur, antimony and/or selenium. Semiconductor material systems mayalso be employed, including materials and alloys such as silicon,silicon-germanium, and germanium, or of Group III-V compounds (whereGroup III includes elements such as Germanium, Aluminum, Indium, etc.and Group V includes elements such as nitrogen, phosphorus, arsenic,etc.). Such semiconductor systems are suitable for embodiments of thisinvention which employ epitaxial growth methods.

Concerning Film C (the side cladding film), which itself as describedabove can comprise an alternating stack of two different materials withrefractive indices n_(H) and n_(L), it may be deposited on the substrateeither by epitaxial growth, sputtering, metalorganic chemical vapordeposition (MOCVD), vacuum evaporation, plasma-enhanced chemical vapordeposition (PECVD), or low-pressure chemical vapor deposition (LPCVD),inductively-coupled plasma-enhanced chemical vapor deposition(ICP-PECVD), or any other technique of depositing materials withsuitable refractive indices in an alternating combination as prescribed,typically with low interface roughness <50 angstroms and in thicknessesranging from 5-300 nm for each layer.

Concerning Film B (the core film), it typically comprises a materialthat can be deposited into the etched trenches of Film C, such that aconformal and smooth coating of the sidewalls is achieved without voidsor inhomogeneities. Additionally, it is desirable that it is opticallyisotropic in order to maintain the proper relationship betweenrefractive indices of the core and cladding. To suitably deposit Film B,vacuum evaporation methods may be applied assuming the substrate isrotated or translated during the process in order to expose the etchedsidewalls of trenches to incoming material evenly. Chemical vapordeposition methods as described above are all generally suitable as theymay provide conformal coating on sidewalls. Epitaxial growth may also beapplied, provided that low-stress growth on the etched surfaces of FilmC and potentially the exposed Film D (upper cladding) is possible.

Concerning Film A, the lower cladding film, it can either be the samematerial as the substrate if the refractive index is suitable, or it maybe achieved by partial oxidation of the substrate (as in the oxidationof silicon to achieve silicon dioxide), or by any deposition meansmentioned prior. If single-crystal growth quality is required (in thecase of epitaxial growth or MOCVD), it typically consists of a suitablesingle-crystal material upon which to grow the constituents of Film C.

Concerning Film D, a similar characteristic of being able to conformallycoat steep sidewalls (similar to Film B) is typically desirable, and thesame suite of deposition methods and characteristics applies to Film Das to Film B. For both Films B and D, the depositable thickness shouldbe able to exceed the etched trench depth in order to achieve thedesired optical properties.

Although many such combinations may be possible as described above, itis useful to detail particular embodiments that are readily envisioned.For example, in one specific arrangement, Films A and D may comprisesilicon dioxide, Film B silicon oxynitride (with a suitable compositionto achieve the refractive index requirements stated earlier), and Film Ccomprising alternating films of silicon nitride (to achieve n_(H)) andsilicon dioxide (to achieve n_(L)). Films A, B, C, and D may all bedeposited by any means of chemical vapor deposition (excluding MOCVD).

Concerning the handle wafer, it may comprise any mechanically stablesemiconductor or dielectric material, but advantageously either silicondioxide or silicon as they are generally more affordable to manufacture.However, this invention represents a self-contained system of threelayers (lower cladding, device layer, and upper cladding) that can inprinciple augment any substrate underneath. The substrate itself mayalready possess a variety of materials and devices on its top surfaceprior to the addition of the embodied technology. For example, thesubstrate can comprise an integrated silicon photonic chip possessingsilicon waveguides and metal interconnect features. Typically, the onlyrequirements set forth prior to the addition of this technology is thatthe top surface is planarized or flat, that it provides sufficientadherence to subsequent films that are deposited, and that none of itsmaterials break down in the process of depositing films A-D. An exampleof a suitable arrangement for the case of augmenting a pre-processedsubstrate is given in FIG. 27.

Those skilled in the art will recognize that the arrangement andcomposition of the pre-processed substrate is not limited to thatpictured. Other relevant arrangements may include a photonic integratedcircuit comprising Indium Phosphide- or Gallium Arsenide-based photonicdevices. Interfacing of the pre-processed substrate with the augmentedlayer of this invention may be achieved, for example, by various meanssuch as grating couplers, tapered waveguide directional couplers, orangled reflectors, which are well-known in this field of research.

EXAMPLES

Disclosed embodiments of the invention are further illustrated by thefollowing specific Examples, which should not be construed as limitingthe scope or content of this Disclosure in any way.

Example 1

In this Example, dielectric materials were chosen to implement thedevice on a silicon substrate. Silicon dioxide was selected for theupper cladding (Film A), lower cladding (Film D) and one of the layersof film C (side cladding), layer n_(L). Silicon nitride was selected forother layer n_(H) of film C (side cladding); and silicon oxynitride wasselected for the core film (Film B). Low slab propagation losses weremeasured in the multilayer stacks as well as the silicon oxynitridelayers.

Waveguides were fabricated using these material for Films A-D, andtested at a 633 nm wavelength. FIG. 28A shows low-loss propagation asobserved from the top surface of a chip having a disclosed TM-onlywaveguide; and FIG. 28B shows low-loss propagation as observed from thetop surface of the chip having a disclosed TE-only waveguide. Theseresults validate that the disclosed multilayer structure exhibits theanisotropy that provides birefringence to achieve polarization-selectivewaveguiding.

A test embodiment of a PBS was fabricated using these same materialselections. The PBS showed good efficiency at routing each polarizationinto the desired output channel. Additionally, negligible crosstalk waspresent. The device was tested at 633 nm and 1110 nm wavelengths forboth polarization inputs. An estimated extinction ratio >10 dB andinsertion losses of <1.2 dB were achieved for both polarizations at bothwavelengths, confirming the expected broadband performance.

FIG. 29 includes top and bottom are scanned digital optical images oflight exiting the ports of a PBS according to disclosed aspects. On theright side of the top and bottom digital images show light exiting theTE and TM output ports. On the left side of the top and bottom imagesthere is included a “reference” port with no PBS, showing the fractionof power routed to each port. The similar brightness of light from eachof the output ports as the corresponding reference port is indicative ofvery low losses.

Those skilled in the art to which this Disclosure relates willappreciate that many other embodiments and variations of embodiments arepossible within the scope of the claimed invention, and furtheradditions, deletions, substitutions and modifications may be made to thedescribed embodiments without departing from the scope of thisDisclosure.

1. A photonic device for guiding light having a wavelength (k),comprising: a lower cladding layer on a substrate; a device layerextending in a first direction that is parallel to an in-plane directionof the substrate that is perpendicular to a thickness dimension of thesubstrate disposed on the lower cladding layer, the device layercomprising a first side cladding film and a second side cladding film,and a core film between the first and the second side cladding film; andan upper cladding layer disposed on the device layer, wherein the corefilm comprises one of (1) a material having an index of refraction n_(M)and (2) alternating layers of a first material having a first index ofrefraction and second material having a second index of refraction thatis different than the first material, the alternating layers arranged ina periodic structure that has a periodicity oriented perpendicular tothe in-plane direction of the substrate wherein the alternating layersprovide an effective index of refraction for TE polarized light n_(TE)and a different effective index of refraction for TM polarized lightn_(TM), wherein each of the first side cladding film and the second sidecladding film comprise the other of the material having the index ofrefraction n_(M) and the alternating layers, wherein n_(TM)<n_(M)<n_(TE)at the wavelength λ, and wherein the index of refraction of the uppercladding layer and the lower cladding layer is less than n_(TE), n_(M)and n_(TE) at the wavelength λ.
 2. The photonic device of claim 1,wherein the difference between n_(TM) and n_(M) has a magnitude lessthan plus or minus 30% of the difference between n_(M) and n_(TE), atthe wavelength λ.
 3. The photonic device of claim 2, wherein thedifference between n_(TM) and n_(TE) is in a range of 0.01 to 0.8. 4.The photonic device of claim 1, wherein the core film comprises thematerial having an index of refraction n_(M).
 5. The photonic device ofclaim 1, wherein the core film comprises the alternating layers.
 6. Thephotonic device of claim 1, further comprising an input bi-polarized E/Mwaveguide supporting both transverse-Electric and transverse-Magneticpolarization-modes optically coupled to the core film.
 7. The photonicdevice of claim 6, wherein the input E/M waveguide comprises: an inputE/M waveguide lower cladding layer; an input E/M waveguide device layerdisposed on the input E/M waveguide lower cladding layer, the input E/Mwaveguide device layer comprising an input E/M waveguide first claddingfilm and an input E/M waveguide second cladding film, and the input coreextending in the first direction between the input E/M waveguide firstcladding film and the input E/M waveguide second cladding film; and aninput E/M waveguide upper cladding layer disposed on the input E/Mwaveguide device layer, the input core comprising a material havingindex of refraction greater than the index of refraction of the inputE/M waveguide lower cladding layer, the input E/M waveguide firstcladding film, the input E/M waveguide second cladding film and theupper cladding layer at the wavelength λ.
 8. The photonic device ofclaim 7, wherein the photonic device is a polarizer, the polarizerfurther comprising: a first transition region disposed between the inputwaveguide and a polarization-dependent region where a width of the firstcladding film is disposed between the core film and a width of the inputE/M waveguide first cladding film, and a width of the second claddingfilm is disposed between the core film and a width of the input E/Mwaveguide second cladding film, wherein, in the first transition region,(1) the width of the first cladding film increases and the width of theinput E/M waveguide first cladding film decreases along the firstdirection and (2) the width of the second cladding film increases andthe width of the input E/M waveguide second cladding film decreasesalong the first direction.
 9. The photonic device of claim 8, furthercomprising an output E/M waveguide having an output core opticallycoupled to the core film, at an opposite end from the input E/Mwaveguide.
 10. The photonic device of claim 9, wherein the output E/Mwaveguide comprises: an output E/M waveguide lower cladding layer; anoutput E/M waveguide device layer disposed on the output E/M waveguidelower cladding layer, the output E/M waveguide device layer comprisingan output E/M waveguide first cladding film and an output E/M waveguidesecond cladding film, and the input core extending in the firstdirection between the output E/M waveguide first cladding film and theoutput E/M waveguide second cladding film; and an output E/M waveguideupper cladding layer disposed on the output E/M waveguide device layer,the output core comprising a material having index of refraction greaterthan the index of refraction of the output E/M waveguide lower claddinglayer, the output E/M waveguide first cladding film, the output E/Mwaveguide second cladding film and the upper cladding layer at thewavelength λ.
 11. The photonic device of claim 10, the device furthercomprising: a second transition region disposed between the outputwaveguide and the polarization-dependent region where a width of thefirst cladding film is disposed between the core film and a width of theoutput E/M waveguide first cladding film, and a width of the secondcladding film is disposed between the core film and a width of theoutput E/M waveguide second cladding film, wherein in the secondtransition region, (1) the width of the first cladding film decreasesand the width of the output E/M waveguide first cladding film increasesalong the first direction and (2) the width of the second cladding filmdecreases and the width of the output E/M waveguide second cladding filmincreases along the first direction.
 12. The photonic device of claim11, wherein the polarizer is a TE-pass polarizer.
 13. The photonicdevice of claim 11, wherein the polarizer is a TM-pass polarizer.
 14. Aphotonic device for guiding light in a first direction having awavelength λ, comprising: a bus waveguide; a second waveguide having acore extending in a first direction that is parallel to an in-planedirection of the bus waveguide that is perpendicular to a thicknessdimension of the bus waveguide characterized by a width equal to Wtransverse to the core and the bus waveguide, the second waveguidehaving a first tapered region proximate the bus waveguide in which thewidth is reduced along the first direction, and a second tapered regionproximate the bus waveguide in which the width is increased along thefirst direction back to W, the second waveguide being evanescentlycoupled to the bus waveguide between the first tapered region and thesecond tapered region; and a cladding material disposed between the buswaveguide and the second waveguide; and the second waveguide comprisingone of (1) a material having an index of refraction n_(M) and (2)alternating layers of a first material having a first index ofrefraction and second material having a second index of refraction thatis different than the first material, the alternating layers arranged ina periodic structure that has a periodicity oriented perpendicular tothe in-plane direction of the bus waveguide wherein the alternatinglayers provide an effective index of refraction for TE polarized lightn_(TE) and an effective index of refraction for TM polarized lightn_(TE), the bus waveguide comprising the other of (1) the materialhaving an index of refraction n_(TM) and (2) the alternating layers,where n_(TM)<n_(M)<n_(TE) at the wavelength λ, and the index ofrefraction of the upper cladding and the lower cladding is less thann_(TM), n_(M) and n_(TE) at the wavelength λ.
 15. The photonic device ofclaim 14, wherein the second waveguide is a ring waveguide.
 16. Thephotonic device of claim 14, wherein the bus waveguide comprises thematerial having an index of refraction n_(M).
 17. The photonic device ofclaim 14, wherein the bus waveguide comprises the alternating layers.18. A photonic device for guiding light in a first direction and todivide the light into a first output having only TE polarized light anda second output having only TM polarized light, the light having awavelength λ, the photonic device comprising: a lower side claddinglayer on a substrate; a device layer extending in a first direction thatis parallel to an in-plane direction of the substrate that isperpendicular to a thickness dimension of the substrate disposed on thelower side cladding layer, an upper side cladding layer disposed on thedevice layer, wherein the device layer comprises a core film extendingin the first direction between a first side cladding film and a secondside cladding film, the core film comprising a transition region and aseparation region in the transition region, the core film comprises aninput core having a first width transverse to the first direction and atransition core contacting the input core and the transition core havinga second width that increases along the first direction until core filmhas a width equal to the 1.3 to 3.0 times the input core width; and inthe separation region, the input core is separated from the transitioncore by a separation distance that increases along the first directionto a size that prevents coupling of the light of the wavelength λbetween the input core and the transition core; the input corecomprising one of (1) a material having an index of refraction n_(M) and(2) alternating layers of a first material having a first index ofrefraction and second material having a second index of refraction thatis different than the first material, the alternating layers arranged ina periodic structure that has a periodicity oriented perpendicular tothe in-plane direction of the substrate having an effective index ofrefraction for the TE polarized light n_(TE) and a different effectiveindex of refraction for the TM polarized light n_(TM), the transitioncore comprising the other of (1) the material having an index ofrefraction n_(M) and (2) the alternating layers, wheren_(TM)<n_(TM)<n_(TE) at the wavelength λ, and each of the upper claddinglayer, the lower side cladding layer, the first cladding film, thesecond cladding film and a separation cladding film have an index ofrefraction less than each of n_(TM), n_(M), n_(TE) at the wavelength λ,whereby the input core forms the output of only a first of TE polarizedlight and the TM polarized light, and the transition core forms anoutput of only a second of the TE polarized light and the TM polarizedlight.
 19. The photonic device of claim 18, wherein the input core andthe transition core are separated from one another in the separationregion at an angle from 0.1 to 10 degrees.
 20. The photonic device ofclaim 18, wherein the first width equals the second width where thetransition region and the separation region meet.
 21. The photonicdevice of claim 18, further comprising a second transition region wherethe transition core has a constant width and the input core has a widththat increases along the first direction.